1. Field of the Invention
The present invention generally relates to a method of fabricating a current control device capable of regulating current flow via compression and expansion of a variably resistive composite.
2. Related Arts
Mechanical circuit breakers are best described as a switch wherein a contact alters the electrical impedance between a source and a load. Mechanical breakers include a snap-action bimetal-contact, a mechanical latch/spring, or an expansion wire. Presently known devices are neither gap-less nor shock resistant and therefore prone to chatter and arcing which cause substantial problems in many high-voltage applications.
Variably conductive composites are applicable to current control devices. Compositions include positive temperature coefficient resistive (PTCR), polymer current limiter (PCL), and piezoresistive formulations. PTCR and PCL applications and compositions and piezoresistive compositions are described in the related arts.
Anthony, U.S. Pat. No. 6,157,528, describes and claims a polymer fuse composed of a PTCR composition exhibiting temperature-dependent resistivity wherein low resistivity results below and high resistivity results above a transition temperature.
PTCR composites are composed of a conductive filler within a polymer matrix and an optional non-conductive filler. Chandler et al., U.S. Pat. No. 5,378,407, describes and claims a PTCR composite including a crystalline polymer matrix, nickel conductive filler, and dehydrated metal-oxide non-conductive filler. Sadhir et al., U.S. Pat. No. 5,968,419, describes and claims a PTCR composite including an amorphous polymer matrix, thermoplastic non-conductive filler, and conductive filler. During a fault, the composite heats thereby increasing volumetrically until there is sufficient separation between particles composing the conductive filler so as to interrupt current flow. Thereafter, the composite cools and shrinks restoring conduction. This self-restoring feature limits PTCR compositions to temporary interrupt devices.
PCL composites, like PTCR compositions, are a mixture of a conductive filler and a polymer. However, PCL composites are conductive when compressed and interrupt current flow by polymer decomposition. For example, Duggal et al., U.S. Pat. No. 5,614,881, describes a composite including a pyrolytic-polymer matrix and electrically conductive filler. During a fault, temperature within the composite increases causing limited decomposition and evolution of gaseous products. Current flow is interrupted when separation occurs between at least one electrode and conductive polymer. Gap dependent interrupt promotes arcing and arc related transients. Furthermore, static compression of the composites increases time-to-interrupt by damping gap formation. Neither PTCR nor PCL applications provide for the dynamically-tunable compression of a composite in response to electrical load conditions.
Piezoresistive composites, also referred to as pressure conduction composites, exhibit pressure-sensitive resistivity rather than temperature or decomposition dependence. Harden et al., U.S. Pat. No. 4,028,276, describes piezoresistive composites composed of an electrically conductive fill within a polymer matrix with an optional additive. Conductive particles comprising the conductive filler 2 are dispersed and separated within the non-conductive matrix 3, as represented in FIGS. 1a and 1c. Consequently, piezoresistive composites are inherently resistive becoming less resistive and more conductive when compressed by a force 9. Compression reduces the distance between conductive particles thereby forming a conductive pathway, as represented in FIGS. 1b and 1d. The composite is again resistive after the compressive force is removed. However, known piezoresistive compositions resist compression.
Pressure-based interrupt facilitates a more rapid regulation of current flow as compared to PTCR and PCL systems. Temperature dependent interrupt is slowed by the poor thermal conduction properties of the polymer matrix. Decomposition dependent interrupt is a two-step process requiring both gas evolution and physical separation between electrode and composite. Furthermore, decomposition limits the life cycle of a composition.
Active materials, including but not limited to piezoelectric, piezoceramic, electrostrictive, magnetostrictive, and shape-memory alloy materials, are ideally suited to the controlled compression of piezoresistive composites, thereby achieving rapid and/or precise changes to resistivity. Active materials facilitate rapid movement by mechanically distorting or resonating when energized. High-bandwidth active materials are both sufficiently robust to exert a large mechanical force and sufficiently precise to controllably adjust force magnitude.
Accordingly, what is required is a method of fabricating a current control device having a pressure-dependent conduction therein that is less resistive to compression and which avoids the arcing and transient spikes typical of the related arts.